Naturally occurring antiphospholipid antibodies (aPLs) are commonly associated with disruption of pregnancy, characterized by preeclampsia, intrauterine growth restriction, or death of the fetus. APLs against phosphatidylserine (PS)- and cardiolipin (CL)- dependent antigens are most commonly associated with pregnancy complications. Our previous studies have demonstrated that mouse monoclonal and human polyclonal antiphosphatidylserine antibodies (aPS) bind directly to antigens on trophoblast, both villous and extravillous. These PS-dependent antigens are expressed in a differentiation-related manner on the trophoblast. We have also shown that monoclonal aPS inhibits in vitro assays for intertrophoblastic fusion, hormone production, and trophoblastic invasion, as well a induces IUGR in pregnant mice. We hypothesize that during differentiation PS-dependent antigens are externalized on trophoblastic cells and that aPS inhibits trophoblastic activities that are necessary for successful pregnancy. Our long-range goals are to understand the mechanisms that regulate the externalization of PS- dependent antigens, the role of external PS in trophoblast development, and the mechanism by which aPLs interfere with this process. In this competing continuation, we will 1) determine the specificity of aPLs and the nature of their antigens, 2) determine the mechanisms that control trophoblastic PS externalization and its significance to differentiation, and 3) determine the mechanisms of aPLs~ effects on trophoblast function. These later studies will include localization of aPL binding to trophoblast, determining the effects of aPLs on trophoblast apoptosis, and evaluating the effects of aPLs on in vitro intertrophoblastic fusion, the production of pregnancy-related hormones, trophoblast invasion of extracellular matrix, trophoblast-mediated control of maternal coagulation, and trophoblastic signal transduction through protein kinase C. These studies should clarify the role of phospholipid externalization in trophoblast differentiation and the mechanisms by which aPLs affect those functions.